[PATCH-block v2 0/3] blk-cgroup: Fix potential UAF & miscellaneous cleanup

[PATCH-block v2 0/3] blk-cgroup: Fix potential UAF & miscellaneous cleanup

[Waiman Long]

 v2:
  - Remove unnecessary rcu_read_{lock|unlock} from
    cgroup_rstat_css_cpu_flush() in patch 3.

It was found that blkcg_destroy_blkgs() may be called with all blkcg
references gone. This may potentially cause user-after-free and so
should be fixed. The last 2 patches are miscellaneous cleanups of
commit 3b8cc6298724 ("blk-cgroup: Optimize blkcg_rstat_flush()").

Waiman Long (3):
  bdi, blk-cgroup: Fix potential UAF of blkcg
  blk-cgroup: Don't flush a blkg if destroyed
  blk-cgroup: Flush stats at blkgs destruction path

 block/blk-cgroup.c     | 26 ++++++++++++++++++++++++++
 include/linux/cgroup.h |  1 +
 kernel/cgroup/rstat.c  | 18 ++++++++++++++++++
 mm/backing-dev.c       |  8 ++++++--
 4 files changed, 51 insertions(+), 2 deletions(-)

-- 
2.31.1

[PATCH-block v2 1/3] bdi, blk-cgroup: Fix potential UAF of blkcg

[Waiman Long]

Commit 59b57717fff8 ("blkcg: delay blkg destruction until after
writeback has finished") delayed call to blkcg_destroy_blkgs() to
cgwb_release_workfn(). However, it is done after a css_put() of blkcg
which may be the final put that causes the blkcg to be freed as RCU
read lock isn't held.

Another place where blkcg_destroy_blkgs() can be called indirectly via
blkcg_unpin_online() is from the offline_css() function called from
css_killed_work_fn(). Over there, the potentially final css_put() call
is issued after offline_css().

By adding a css_tryget() into blkcg_destroy_blkgs() and warning its
failure, the following stack trace was produced in a test system on
bootup.

[   34.254240] RIP: 0010:blkcg_destroy_blkgs+0x16a/0x1a0
      :
[   34.339943] Call Trace:
[   34.344510]  blkcg_unpin_online+0x38/0x60
[   34.348523]  cgwb_release_workfn+0x6a/0x200
[   34.352708]  process_one_work+0x1e5/0x3b0
[   34.360758]  worker_thread+0x50/0x3a0
[   34.368447]  kthread+0xd9/0x100
[   34.376386]  ret_from_fork+0x22/0x30

This confirms that a potential UAF situation can really happen in
cgwb_release_workfn().

Fix that by delaying the css_put() until after the blkcg_unpin_online()
call. Also use css_tryget() in blkcg_destroy_blkgs() and issue a warning
if css_tryget() fails.

The reproducing system can no longer produce a warning with this patch.
All the runnable block/0* tests including block/027 were run successfully
without failure.

Fixes: 59b57717fff8 ("blkcg: delay blkg destruction until after writeback has finished")
Suggested-by: Michal Koutný <mkoutny@suse.com>
Reported-by: Yi Zhang <yi.zhang@redhat.com>
Signed-off-by: Waiman Long <longman@redhat.com>
---
 block/blk-cgroup.c | 8 ++++++++
 mm/backing-dev.c   | 8 ++++++--
 2 files changed, 14 insertions(+), 2 deletions(-)

diff --git a/block/blk-cgroup.c b/block/blk-cgroup.c
index 1bb939d3b793..21cc88349f21 100644
--- a/block/blk-cgroup.c
+++ b/block/blk-cgroup.c
@@ -1084,6 +1084,13 @@ struct list_head *blkcg_get_cgwb_list(struct cgroup_subsys_state *css)
  */
 static void blkcg_destroy_blkgs(struct blkcg *blkcg)
 {
+	/*
+	 * blkcg_destroy_blkgs() shouldn't be called with all the blkcg
+	 * references gone.
+	 */
+	if (WARN_ON_ONCE(!css_tryget(&blkcg->css)))
+		return;
+
 	might_sleep();
 
 	spin_lock_irq(&blkcg->lock);
@@ -1110,6 +1117,7 @@ static void blkcg_destroy_blkgs(struct blkcg *blkcg)
 	}
 
 	spin_unlock_irq(&blkcg->lock);
+	css_put(&blkcg->css);
 }
 
 /**
diff --git a/mm/backing-dev.c b/mm/backing-dev.c
index c30419a5e119..36f75b072325 100644
--- a/mm/backing-dev.c
+++ b/mm/backing-dev.c
@@ -390,11 +390,15 @@ static void cgwb_release_workfn(struct work_struct *work)
 	wb_shutdown(wb);
 
 	css_put(wb->memcg_css);
-	css_put(wb->blkcg_css);
 	mutex_unlock(&wb->bdi->cgwb_release_mutex);
 
-	/* triggers blkg destruction if no online users left */
+	/*
+	 * Triggers blkg destruction if no online users left
+	 * The final blkcg css_put() has to be done after blkcg_unpin_online()
+	 * to avoid use-after-free.
+	 */
 	blkcg_unpin_online(wb->blkcg_css);
+	css_put(wb->blkcg_css);
 
 	fprop_local_destroy_percpu(&wb->memcg_completions);
 
-- 
2.31.1

[PATCH-block v2 2/3] blk-cgroup: Don't flush a blkg if destroyed

[Waiman Long]

Before commit 3b8cc6298724 ("blk-cgroup: Optimize blkcg_rstat_flush()"),
blkg's stats is only flushed if they are online. In addition, the
stat flushing of blkgs in blkcg_rstat_flush() includes propagating
the rstat data to its parent. However, if a blkg has been destroyed
(offline), the validity of its parent may be questionable. For safety,
revert back to the old behavior by ignoring offline blkg's.

Signed-off-by: Waiman Long <longman@redhat.com>
---
 block/blk-cgroup.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/block/blk-cgroup.c b/block/blk-cgroup.c
index 21cc88349f21..c466aef0d467 100644
--- a/block/blk-cgroup.c
+++ b/block/blk-cgroup.c
@@ -885,6 +885,12 @@ static void blkcg_rstat_flush(struct cgroup_subsys_state *css, int cpu)
 
 		WRITE_ONCE(bisc->lqueued, false);
 
+		/* Don't flush its stats if blkg is offline */
+		if (unlikely(!blkg->online)) {
+			percpu_ref_put(&blkg->refcnt);
+			continue;
+		}
+
 		/* fetch the current per-cpu values */
 		do {
 			seq = u64_stats_fetch_begin(&bisc->sync);
-- 
2.31.1

[PATCH-block v2 3/3] blk-cgroup: Flush stats at blkgs destruction path

[Waiman Long]

As noted by Michal, the blkg_iostat_set's in the lockless list
hold reference to blkg's to protect against their removal. Those
blkg's hold reference to blkcg. When a cgroup is being destroyed,
cgroup_rstat_flush() is only called at css_release_work_fn() which is
called when the blkcg reference count reaches 0. This circular dependency
will prevent blkcg from being freed until some other events cause
cgroup_rstat_flush() to be called to flush out the pending blkcg stats.

To prevent this delayed blkcg removal, add a new cgroup_rstat_css_flush()
function to flush stats for a given css and cpu and call it at the blkgs
destruction path, blkcg_destroy_blkgs(), whenever there are still some
pending stats to be flushed. This will ensure that blkcg reference
count can reach 0 ASAP.

Signed-off-by: Waiman Long <longman@redhat.com>
---
 block/blk-cgroup.c     | 12 ++++++++++++
 include/linux/cgroup.h |  1 +
 kernel/cgroup/rstat.c  | 18 ++++++++++++++++++
 3 files changed, 31 insertions(+)

diff --git a/block/blk-cgroup.c b/block/blk-cgroup.c
index c466aef0d467..534f3baeb84a 100644
--- a/block/blk-cgroup.c
+++ b/block/blk-cgroup.c
@@ -1090,6 +1090,8 @@ struct list_head *blkcg_get_cgwb_list(struct cgroup_subsys_state *css)
  */
 static void blkcg_destroy_blkgs(struct blkcg *blkcg)
 {
+	int cpu;
+
 	/*
 	 * blkcg_destroy_blkgs() shouldn't be called with all the blkcg
 	 * references gone.
@@ -1099,6 +1101,16 @@ static void blkcg_destroy_blkgs(struct blkcg *blkcg)
 
 	might_sleep();
 
+	/*
+	 * Flush all the non-empty percpu lockless lists.
+	 */
+	for_each_possible_cpu(cpu) {
+		struct llist_head *lhead = per_cpu_ptr(blkcg->lhead, cpu);
+
+		if (!llist_empty(lhead))
+			cgroup_rstat_css_cpu_flush(&blkcg->css, cpu);
+	}
+
 	spin_lock_irq(&blkcg->lock);
 
 	while (!hlist_empty(&blkcg->blkg_list)) {
diff --git a/include/linux/cgroup.h b/include/linux/cgroup.h
index 528bd44b59e2..6c4e66b3fa84 100644
--- a/include/linux/cgroup.h
+++ b/include/linux/cgroup.h
@@ -766,6 +766,7 @@ void cgroup_rstat_flush(struct cgroup *cgrp);
 void cgroup_rstat_flush_irqsafe(struct cgroup *cgrp);
 void cgroup_rstat_flush_hold(struct cgroup *cgrp);
 void cgroup_rstat_flush_release(void);
+void cgroup_rstat_css_cpu_flush(struct cgroup_subsys_state *css, int cpu);
 
 /*
  * Basic resource stats.
diff --git a/kernel/cgroup/rstat.c b/kernel/cgroup/rstat.c
index 793ecff29038..2e44be44351f 100644
--- a/kernel/cgroup/rstat.c
+++ b/kernel/cgroup/rstat.c
@@ -281,6 +281,24 @@ void cgroup_rstat_flush_release(void)
 	spin_unlock_irq(&cgroup_rstat_lock);
 }
 
+/**
+ * cgroup_rstat_css_cpu_flush - flush stats for the given css and cpu
+ * @css: target css to be flush
+ * @cpu: the cpu that holds the stats to be flush
+ *
+ * A lightweight rstat flush operation for a given css and cpu.
+ * Only the cpu_lock is being held for mutual exclusion, the cgroup_rstat_lock
+ * isn't used.
+ */
+void cgroup_rstat_css_cpu_flush(struct cgroup_subsys_state *css, int cpu)
+{
+	raw_spinlock_t *cpu_lock = per_cpu_ptr(&cgroup_rstat_cpu_lock, cpu);
+
+	raw_spin_lock_irq(cpu_lock);
+	css->ss->css_rstat_flush(css, cpu);
+	raw_spin_unlock_irq(cpu_lock);
+}
+
 int cgroup_rstat_init(struct cgroup *cgrp)
 {
 	int cpu;
-- 
2.31.1

Re: [PATCH-block v2 2/3] blk-cgroup: Don't flush a blkg if destroyed

[Michal Koutný]

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Hello.

On Sun, Dec 11, 2022 at 05:20:57PM -0500, Waiman Long <longman@redhat.com> wrote:
> Before commit 3b8cc6298724 ("blk-cgroup: Optimize blkcg_rstat_flush()"),
> blkg's stats is only flushed if they are online.

I'm not sure I follow -- css_release_work_fn/cgroup_rstat_flush may be
called on an offlined blkcg (offlined!=released). There's no invariant
ensuring offlined blkcg won't be flushed. (There is only current
situation when there is no reader of io data that'd need them flushed
[1].)

> In addition, the stat flushing of blkgs in blkcg_rstat_flush()
> includes propagating the rstat data to its parent. However, if a blkg
> has been destroyed (offline), the validity of its parent may be
> questionable.

Parents won't be freed (neither offlined) before children (see
css_killed_work_fn). It should be regularly OK to pass data into a
parent of an offlined blkcg.

> For safety, revert back to the old behavior by ignoring offline
> blkg's.

I don't know if this is a good reasoning. If you argue that offlined
children needn't be taken into parent's account, then I think it's more
efficient to exclude the offlined blkcgs from update. (With the caveat I
have in [1].)

Regards,
Michal

[1] https://lore.kernel.org/r/YqEfNgUc8jxlAq8D@blackbook/

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Re: [PATCH-block v2 2/3] blk-cgroup: Don't flush a blkg if destroyed

[Waiman Long]

On 12/12/22 07:59, Michal Koutný wrote:
> Hello.
>
> On Sun, Dec 11, 2022 at 05:20:57PM -0500, Waiman Long <longman@redhat.com> wrote:
>> Before commit 3b8cc6298724 ("blk-cgroup: Optimize blkcg_rstat_flush()"),
>> blkg's stats is only flushed if they are online.
> I'm not sure I follow -- css_release_work_fn/cgroup_rstat_flush may be
> called on an offlined blkcg (offlined!=released). There's no invariant
> ensuring offlined blkcg won't be flushed. (There is only current
> situation when there is no reader of io data that'd need them flushed
> [1].)
The original cgroup_rstat_flush() iterates the list of blkg's in the 
blkg_list. A blkg will be removed from the list when it is offlined. 
This patch just reverts its behavior to that previous behavior.
>
>> In addition, the stat flushing of blkgs in blkcg_rstat_flush()
>> includes propagating the rstat data to its parent. However, if a blkg
>> has been destroyed (offline), the validity of its parent may be
>> questionable.
> Parents won't be freed (neither offlined) before children (see
> css_killed_work_fn). It should be regularly OK to pass data into a
> parent of an offlined blkcg.
I guess it is likely to be safe to flush an offline blkg. I am just 
being conservative in case there is any corner case where it may be a 
problem which I haven't foreseen.
>
>> For safety, revert back to the old behavior by ignoring offline
>> blkg's.
> I don't know if this is a good reasoning. If you argue that offlined
> children needn't be taken into parent's account, then I think it's more
> efficient to exclude the offlined blkcgs from update. (With the caveat I
> have in [1].)

It is possible that a blkg may be updated before it becomes offline, but 
the flush isn't done in time before that happens. The next patch will 
catch some of that.

Cheers,
Longman

Re: [PATCH-block v2 1/3] bdi, blk-cgroup: Fix potential UAF of blkcg

[Tejun Heo]

On Sun, Dec 11, 2022 at 05:20:56PM -0500, Waiman Long wrote:
>  static void blkcg_destroy_blkgs(struct blkcg *blkcg)
>  {
> +	/*
> +	 * blkcg_destroy_blkgs() shouldn't be called with all the blkcg
> +	 * references gone.
> +	 */
> +	if (WARN_ON_ONCE(!css_tryget(&blkcg->css)))
> +		return;

Wouldn't it make more sense to use percpu_ref_is_zero()? It's not like the
obtained extra reference does anything, right?

Thanks.

-- 
tejun

Re: [PATCH-block v2 2/3] blk-cgroup: Don't flush a blkg if destroyed

[Tejun Heo]

On Mon, Dec 12, 2022 at 01:59:53PM +0100, Michal Koutný wrote:
> Hello.
> 
> On Sun, Dec 11, 2022 at 05:20:57PM -0500, Waiman Long <longman@redhat.com> wrote:
> > Before commit 3b8cc6298724 ("blk-cgroup: Optimize blkcg_rstat_flush()"),
> > blkg's stats is only flushed if they are online.
> 
> I'm not sure I follow -- css_release_work_fn/cgroup_rstat_flush may be
> called on an offlined blkcg (offlined!=released). There's no invariant
> ensuring offlined blkcg won't be flushed. (There is only current
> situation when there is no reader of io data that'd need them flushed
> [1].)
> 
> > In addition, the stat flushing of blkgs in blkcg_rstat_flush()
> > includes propagating the rstat data to its parent. However, if a blkg
> > has been destroyed (offline), the validity of its parent may be
> > questionable.
> 
> Parents won't be freed (neither offlined) before children (see
> css_killed_work_fn). It should be regularly OK to pass data into a
> parent of an offlined blkcg.
> 
> > For safety, revert back to the old behavior by ignoring offline
> > blkg's.
> 
> I don't know if this is a good reasoning. If you argue that offlined
> children needn't be taken into parent's account, then I think it's more
> efficient to exclude the offlined blkcgs from update. (With the caveat I
> have in [1].)

Yeah, I'm not sure about this patch either. Doesn't seem necessary.

Thanks.

-- 
tejun

Re: [PATCH-block v2 1/3] bdi, blk-cgroup: Fix potential UAF of blkcg

[Waiman Long]

On 12/12/22 17:13, Tejun Heo wrote:
> On Sun, Dec 11, 2022 at 05:20:56PM -0500, Waiman Long wrote:
>>   static void blkcg_destroy_blkgs(struct blkcg *blkcg)
>>   {
>> +	/*
>> +	 * blkcg_destroy_blkgs() shouldn't be called with all the blkcg
>> +	 * references gone.
>> +	 */
>> +	if (WARN_ON_ONCE(!css_tryget(&blkcg->css)))
>> +		return;
> Wouldn't it make more sense to use percpu_ref_is_zero()? It's not like the
> obtained extra reference does anything, right?

Yes, that makes sense. Will incorporate the change in the next version.

Thanks,
Longman

Re: [PATCH-block v2 3/3] blk-cgroup: Flush stats at blkgs destruction path

[Tejun Heo]

On Sun, Dec 11, 2022 at 05:20:58PM -0500, Waiman Long wrote:
> As noted by Michal, the blkg_iostat_set's in the lockless list
> hold reference to blkg's to protect against their removal. Those
> blkg's hold reference to blkcg. When a cgroup is being destroyed,
> cgroup_rstat_flush() is only called at css_release_work_fn() which is
> called when the blkcg reference count reaches 0. This circular dependency
> will prevent blkcg from being freed until some other events cause
> cgroup_rstat_flush() to be called to flush out the pending blkcg stats.
> 
> To prevent this delayed blkcg removal, add a new cgroup_rstat_css_flush()
> function to flush stats for a given css and cpu and call it at the blkgs
> destruction path, blkcg_destroy_blkgs(), whenever there are still some
> pending stats to be flushed. This will ensure that blkcg reference
> count can reach 0 ASAP.
> 
> Signed-off-by: Waiman Long <longman@redhat.com>

Acked-by: Tejun Heo <tj@kernel.org>

But a nit below

> +	/*
> +	 * Flush all the non-empty percpu lockless lists.
> +	 */

Can you please explain the deadlock that's being avoided in the above
comment? ie. it should say why this flush is necessary.

> +	for_each_possible_cpu(cpu) {
> +		struct llist_head *lhead = per_cpu_ptr(blkcg->lhead, cpu);
> +
> +		if (!llist_empty(lhead))
> +			cgroup_rstat_css_cpu_flush(&blkcg->css, cpu);
> +	}

Thanks.

-- 
tejun

Re: [PATCH-block v2 3/3] blk-cgroup: Flush stats at blkgs destruction path

[Waiman Long]

On 12/12/22 17:24, Tejun Heo wrote:
> On Sun, Dec 11, 2022 at 05:20:58PM -0500, Waiman Long wrote:
>> As noted by Michal, the blkg_iostat_set's in the lockless list
>> hold reference to blkg's to protect against their removal. Those
>> blkg's hold reference to blkcg. When a cgroup is being destroyed,
>> cgroup_rstat_flush() is only called at css_release_work_fn() which is
>> called when the blkcg reference count reaches 0. This circular dependency
>> will prevent blkcg from being freed until some other events cause
>> cgroup_rstat_flush() to be called to flush out the pending blkcg stats.
>>
>> To prevent this delayed blkcg removal, add a new cgroup_rstat_css_flush()
>> function to flush stats for a given css and cpu and call it at the blkgs
>> destruction path, blkcg_destroy_blkgs(), whenever there are still some
>> pending stats to be flushed. This will ensure that blkcg reference
>> count can reach 0 ASAP.
>>
>> Signed-off-by: Waiman Long <longman@redhat.com>
> Acked-by: Tejun Heo <tj@kernel.org>
>
> But a nit below
>
>> +	/*
>> +	 * Flush all the non-empty percpu lockless lists.
>> +	 */
> Can you please explain the deadlock that's being avoided in the above
> comment? ie. it should say why this flush is necessary.

Sure. I will expand the comment to elaborate a bit more.

Cheers,
Longman

>
>> +	for_each_possible_cpu(cpu) {
>> +		struct llist_head *lhead = per_cpu_ptr(blkcg->lhead, cpu);
>> +
>> +		if (!llist_empty(lhead))
>> +			cgroup_rstat_css_cpu_flush(&blkcg->css, cpu);
>> +	}

Re: [PATCH-block v2 2/3] blk-cgroup: Don't flush a blkg if destroyed

[Waiman Long]

On 12/12/22 17:16, Tejun Heo wrote:
> On Mon, Dec 12, 2022 at 01:59:53PM +0100, Michal Koutný wrote:
>> Hello.
>>
>> On Sun, Dec 11, 2022 at 05:20:57PM -0500, Waiman Long <longman@redhat.com> wrote:
>>> Before commit 3b8cc6298724 ("blk-cgroup: Optimize blkcg_rstat_flush()"),
>>> blkg's stats is only flushed if they are online.
>> I'm not sure I follow -- css_release_work_fn/cgroup_rstat_flush may be
>> called on an offlined blkcg (offlined!=released). There's no invariant
>> ensuring offlined blkcg won't be flushed. (There is only current
>> situation when there is no reader of io data that'd need them flushed
>> [1].)
>>
>>> In addition, the stat flushing of blkgs in blkcg_rstat_flush()
>>> includes propagating the rstat data to its parent. However, if a blkg
>>> has been destroyed (offline), the validity of its parent may be
>>> questionable.
>> Parents won't be freed (neither offlined) before children (see
>> css_killed_work_fn). It should be regularly OK to pass data into a
>> parent of an offlined blkcg.
>>
>>> For safety, revert back to the old behavior by ignoring offline
>>> blkg's.
>> I don't know if this is a good reasoning. If you argue that offlined
>> children needn't be taken into parent's account, then I think it's more
>> efficient to exclude the offlined blkcgs from update. (With the caveat I
>> have in [1].)
> Yeah, I'm not sure about this patch either. Doesn't seem necessary.

I wrote this patch because I am not totally sure it is safe to flush 
offline blkgs. Since both you and Michal don't see any problem with it. 
I am fine taking it out.

Cheers,
Longman